Environmental stresses such as high concentration of salt, drought and cold inhibit plant growth to limit harvest yields in many important agricultural fields. In terms of these stresses, osmotic stress caused by diverse external conditions, for example, high salt, dehydration and cold, is a crucial problem for farmers.
Therefore, with even a slight increase of tolerance to such osmotic stress, it is expected that there would be a considerable improvement in agricultural productivity and yield of crops. For this reason, much research into regulatory mechanisms of plants, with respect to osmotic stress, and regulatory factors involved in the mechanism, have been ongoing. Recent studies have revealed that plants employ elaborate mechanism to partially adapt to osmotic stress, and one of important requirements for such stress-adaptation is the transcriptional activation of a gene encoding a protein necessary to such adaptation (Jang et al., Plant Mol. Biol., 37: 839–847, 1998; Liu et al., Science, 280: 1943–1945, 1998; Pardo et al., Proc. Natl. Acad. Sci. USA, 95: 9681–9686, 1998; Lie et al., Proc. Natl. Acad. Sci. USA, 97: 3730–3734, 2000).
Many genes induced by certain stresses were isolated, and their characteristics have been widely studied, being helpful to understand mechanisms involved in adaptation to osmotic stress. From these studies, it has become clear that there are multiple signaling pathways that lead to induction of osmotic stress responsive genes (Jonak et al., Proc. Natl. Acad. Sci. USA, 93: 11274–11279, 1996; Ishitani et al., Plant Cell 9: 1935–1949, 1997), and these pathways include ABA (abscisic acid)-dependent or ABA-independent pathway (La Rosa et al., Plant Physiol., 85: 174–181, 1987; Savoure et al., Mol. Gen. Genet., 254: 104–109, 1997). In addition, it was discovered that some signaling pathways are common to all osmotic stress conditions, such as high salt, dehydration and cold (Jang et al., Plant Mol. Biol., 37: 839–847, 1998; Liu et al., Science, 280: 1943–1945, 1998; Pardo et al., Proc. Natl. Acad. Sci. USA, 95: 9681–9686, 1998; Lie et al., Proc. Natl. Acad. Sci. USA, 97: 3730–3734, 2000).
As described above, transcriptional control plays a pivotal role in the adaptation responses and is likely to be regulated by specific transcription factors, and several stress-inducible genes encoding such transcription factors or their homolgues have been isolated and characterized (Tague et al., Plant Mol. Biol. 28: 267–279, 1995; Bastola et al., Plant Mol. Biol., 24: 701–713, 1998; Kasuga et al., Nat. Biotechnol., 17: 287–291, 1999; van Der Krol et al., Plant Physiol., 121: 1153–1162, 1999; Nakashima et al., Plant Mol. Biol., 42: 657–665, 2000). Further research provided information that transcription factors encoded by several genes of the discovered genes have a zinc finger motif. Examples of such transcription factors include Atmyb2, ATHB-7, mlip15 (Kusano et al., Mol. Gen. Genet., 248: 507–517, 1995; Soderman et al., Plant J. 10: 375–381,1996), Alfin1 (Bastola et al., Plant Mol. Biol., 24: 701–713, 1998) and AZF1, AZF2 and AZF3 (Sakamoto et al., Gene, 248: 23–32,2000).
Specifically, Atmyb2 in the Arabidopsis plant is induced by dehydration stress, and then bound to a conserved MYB recognition sequence, and induction of an Arabidopsis homeobox gene ATHB-2 is caused by dehydration or abscisic acid treatment. Mlip15 in corn is a transcription factor isolog having a bZIP motif, and is induced at low temperature. Alfin1, a zinc finger protein, is induced by salt stress and has an MsPRP2 promoter-binding site. The Alfin1 protein plays an important role in regulating MsPRP2 expression in the root of alfalfa and consequently contributes to the salt-tolerance in this plant. There is a recent report in which a gene family coding a DRE/CRT binding protein was isolated and characterized (Liu et al., Plant Cell, 10: 1391–1406, 1998). According to the report, such a gene family includes DREB1 (dehydration-inducible element binding protein) and DREB2, which binds to 9 consensus sequences found in promoter regions of a variety of dehydration- or low temperature-inducible genes such as RD29A, Cor6.6 and RD17. However, the transcription factors described above are just a part of the factors constituting a large group of transcription factors involved in a response to osmotic stress.
Since, with respect to such stress-inducible genes, transcription factors can regulate their transcriptions, they may be useful to construct a plant resistant to stress by causing the transcription factors to be overexpressed or inhibited in the plant.